When a battery is mounted in a vehicle or the like driven by an engine, the battery is charged by a rectification output of an AC generator driven by the engine. When the battery is thus charged by the output of the AC generator, a battery charging control device that controls charging of the battery needs to be provided.
The battery charging control device comprises: a rectification and output adjustment circuit that can switch between a state where an output of an AC generator is rectified to supply a charging current to a battery, and a state where the supply of the charging current from the AC generator to the battery is stopped; and a battery voltage adjustment control circuit that controls the rectification and output adjustment circuit so as to supply the charging current from the AC generator to the battery when a voltage across the battery is equal to or lower than a target value, and stop the supply of the charging current to the battery when the voltage across the battery exceeds the target value.
In two-wheel vehicles, outboard motors, agricultural machinery, or the like, a magneto generator is used as an AC generator driven by an engine. When the magneto generator is used, a rectification and output adjustment circuit of generator output short-circuit type is frequently used. The rectification and output adjustment circuit comprises a diode bridge full-wave rectifier circuit that rectifies the output of the AC generator, and a generator output short-circuit switch element which is connected in anti-parallel with each of diodes that form a lower side of a bridge of the diode full-wave rectifier circuit, and a short circuit for short-circuiting the output of the AC generator is constituted by the generator output short-circuit switch element and some of diodes that form the lower side of the diode full-wave rectifier circuit when the generator output short-circuit switch element is turned on.
The generator output short-circuit switch element may be connected in anti-parallel with each of diodes that form an upper side of a bridge of said diode full-wave rectifier circuit. In this case, the short circuit for short-circuiting the output of the AC generator is constituted by the generator output short-circuit switch element and some of diodes that form the upper side of the diode full-wave rectifier circuit when the generator output short-circuit switch element is turned on.
FIG. 2 shows a construction of a conventional battery charging control device using a rectification and output adjustment circuit of generator output short-circuit type. In FIG. 2, a reference numeral 1 denotes a magneto generator driven by a prime motor such as an engine, and 2 denotes a rectification and output adjustment circuit. The rectification and output adjustment circuit 2 is comprised of a diode bridge full-wave rectifier circuit constituted by diodes D1 to D6, and thyristors S1 to S3 as generator output short-circuit switches connected in anti-parallel with the diode D4 to D6 on a lower side of a bridge of the rectifier circuit.
An output of the generator 1 is input into AC input terminals 2u, 2v and 2w of the rectification and output adjustment circuit 2, and a battery 4 is connected between DC output terminals 2a and 2b of the rectification and output adjustment circuit 2 through a fuse 3. An appropriate load (a motor in the shown example) 6 is connected between the output terminals 2a and 2b of the rectification and output adjustment circuit 2 through a load switch 5.
A battery voltage adjustment control circuit 7 comprised of a transistor TR1, a Zener diode ZD1, and resistances R1 to R8 is connected between the DC output terminals 2a and 2b of the rectification and output adjustment circuit 2.
In the battery charging control device in FIG. 2, the Zener diode ZD1 is in off state when a voltage across the battery 4 is equal to or lower than a target value. At this time, no base current passes through the transistor TR1, and thus the transistor TR1 is in off state. When the transistor TR1 is in off state, no trigger signal is provided to the thyristor S1 to S3, thus the output of the AC generator 1 is not short-circuited, and a charging current is supplied from the generator 1 to the battery 4 through the rectification and output adjustment circuit 2. When the load switch 5 is closed, a drive current is supplied from the generator 1 and the battery 4 to the load 6.
When the voltage across the battery 4 exceeds the target value, the Zener diode ZD1 is turned on, thus the base current passes through the transistor TR1 to turn on the transistor, and trigger signals are simultaneously provided from an output terminal side of the rectification and output adjustment circuit 2 to the thyristors S1 to S3 through the transistor TR1 and the resistances R1 to R3. At this time, the output of the generator 1 is short-circuited through any of the thyristors S1 to S3 and any of the diodes D4 to D6 to stop the supply of the charging current to the battery 4. This reduces the voltage across the battery 4. When the voltage across the battery 4 is reduced, the Zener diode ZD1 is turned off to stop the supply of the base current to the transistor TR1, thus the transistor TR1 is turned off to stop the provision of the trigger signals to the thyristors S1 to S3. When the provision of the trigger signals to the thyristors S1 to S3 is stopped, the thyristors S1 to S3 are turned off when an anode current of each thereof becomes equal to or lower than a holding current, and thus the charging current is supplied from the generator 1 to the battery 4 through the rectification and output adjustment circuit 2. Repeating these operations can keep the voltage across the battery 4 around the target value. Such a battery charging control device is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2003-111299.
Conventionally, a large load has been rarely connected to a battery, and most currents output from a battery charging control device have been charging currents of the battery, but in recent years, a relatively large load (an electric load, hereinafter the same) such as a fuel pump used in a fuel injection device or a radiator fan motor has been connected to the battery. For agricultural machinery, a fertilization blower motor or the like is also a large load of a battery.
In order to drive the electric load and charge the battery without a hitch by using the conventional battery charging control device when the large load is connected to the battery, a generator having a large output suitable for the load needs to be used, but actually, the generator having a large output cannot be used in many cases because of a limited mounting position of the generator or limited costs. An insufficient output of the generator prevents sufficient charging of the battery, which may cause the battery to run out.